The present invention relates to an image forming apparatus, for example, a color image forming apparatus in which a pattern-shaped electrostatic latent image is formed on an image carrier using a laser beam source, and a plurality of developing processes by color toners are repeated, and especially to an image forming apparatus in which a plurality of color toners are used so that a color image is formed, at which time an isolated dot or an edge dot is detected, and exposing conditions are respectively changed so that an accurate color image or monochromatic image can be obtained.
A color image forming method in which color toners are superimposed on each other by a conventional electrophotographic method so that an intended color image can be obtained will be explained as follows. At first, as shown in FIG. 1(a), the image carrier E2, which forms a color toner image, is charged at first time by a charger E1 so that the surface potential of the image carrier becomes V.sub.S. Next, only the portion on which an image, that is, a character or a line image, is formed, is imagewise-exposed by the laser beam, as shown in FIG. 1(b) . After an imagewise exposure has been completed, only the exposed portion is discharged and the surface potential V.sub.S is lowered to V.sub.L, and the non-image portion is not discharged. Next, as shown in FIG. 1(c), the image carrier E2 is moved in the arrowed direction and a first color development is conducted by a developing unit D. The charge is eliminated well-type-potential-like by the imagewise exposure as shown in FIG. 1(b), and the first color toner is adhered to the eliminated portion, and developed so that the first color toner image is formed. Next, the second charging is conducted by the charging electrode E1 on the image carrier including the first color toner image portion (re-charging). The surface potential of the image carrier is controlled so that the surface potential becomes almost the same potential as that at the time of the first charging. For explanation, the first color toner image is shown in the following way: the first color toner image is separated from the image carrier E2; and it is shown at the position of the portion of the surface potential V.sub.S. Next, as shown in FIG. 1(e), when the second imagewise exposure is conducted on the first color toner image, the electric charge only at the imagewise exposure portion is eliminated well-type-potential-like in the same manner as that shown in FIG. 1(b) . The light of the second imagewise exposure is transmitted through the first color toner and arrives at the image carrier. The surface potential of that portion becomes (V.sub.L +V.sub.1), in which the potential V.sub.L of the exposed portion is added to the potential V.sub.1 of the toner image. The second color toner is superimposed on the first color toner on the image carrier E2 as shown in FIG. 1(b), and developed by the developing unit D. When the third color is developed, re-charging, imagewise exposure, and the development is repeatedly conducted. The foregoing is a basic process by which a color image is formed on the image carrier when color toners are superimposed.
The above-described color toner image formation by the superimposition of toners is conducted by an apparatus into which image information is inputted through an interface circuit from the outside of the apparatus and in which necessary processing is conducted and the image information is recorded, that is, by an electrophotographic printer in which the image carrier is a photoreceptor. Of course, it may be a color copying apparatus in which a color document is read-out, and after it is converted into an electric signal, the signal is recorded in a recording section and copied. For example, the system which is shown in FIG. 2(a) is composed of a host unit 100 in which a computer (which will be called a CPU, hereinafter) is built-in and a display or the like is provided, a printer controller 101, and a laser printer section 102. The image information such as a character, line, or solid image which is recorded in a recording section in the CPU, and is stored in a magnetic storing means, is outputted to the printer controller. After a video signal from the printer controller 101 is processed for color recording, a recording signal is made, and thereby a semiconductor laser beam is modulated so that the image signal is recorded by the laser printer 102. The system is structured in the following manner: for example, red letters A are successively outputted in a dot image; only a character portion is inputted into the laser printer 102 in a dot signal; and the toner images are formed by the laser printer 102 in order of FIGS. 1(a), (b), (c), (d), (e), and (f).
A pulse width modulation method in which the recording signal is converted into a pulse width in one pixel or a plurality of pixels, and is used for a laser beam modulation signal, and an intensity modulation system in which laser beam radiation intensity is converted into strength, and the size of a recording dot diameter is formed, are used as a semiconductor laser beam modulation system. They are used for the modulation method of the imagewise exposure of the color toner superimposition type. This is convenient when the sizes of the recording dots of the first color and the second color are changed.
For example, when the color of red is prepared by yellow and magenta, it is necessary to adjust the color delicately depending on the cases where yellow is strengthened and magenta is weakened, or the adverse of that, and therefore, the exposure condition is changed and a mixing ratio by the superimposition of color toners is adjusted, so that an intended color image can be obtained. In the above-described color image formation, there is a disadvantage in that: especially at the time of exposing on the first color toner, when exposing is conducted for the second color, the first color toner layer affects it greatly, so that development control of the color toner is difficult.
The above-described color tone control has the following disadvantages: the image signal which is a dot signal, or a dot line, both are character images, or a solid image signal, is outputted from the CPU in FIG. 2, as shown in FIG. 3, to the printer controller 101, and the color tone of any one of them is preferably the same as that of the other two in any image pattern. However, the above-described color tone control method can not always change the color tone to the same degree as the other two. For example, what is called a pulse width modulation system in which lighting time of the exposure unit is changed, will be explained as follows referring to FIG. 3. When the signal is inputted successively from the left in FIG. 3(b), the following operations are conducted: at first, the dot signal is inputted; the solid image signal is inputted after some interval; and the inputted signal is outputted as a video signal from the printer controller 101 to the control section of the laser printer 102.
The above-described video signal is converted into pulse signals by the pulse modulation circuit (not shown in the drawings) in the laser printer 102 control section, and an ON signal corresponding to a dot output signal is generated and a pulse having a constant width is formed. Next, a solid image video signal is inputted into the laser printer 102 after an several interval, and when a pulse signal having a constant width and a constant interval is turned on, the solid image output signal is outputted. For example, when a duty ratio of the exposure unit is adjusted to 50% in order to limit an adhered amount of the first color, and it is adjusted to 100% for the second color, at the time when distribution of the optical energy density which is received by the photoreceptor, a large difference of energy density is generated between the 50% duty ratio in the case of a dot, and the 50% duty ratio in the case of a solid image. That is, in the case of the solid image, an energy amount is a total of the energy of dots which are close to each other. Therefore, an average energy of the solid image is larger than that of an independent isolated dot, one dot line, or an edge area (which is called a dot area, hereinafter) of the solid image area. Accordingly, the difference of the optical energy of the solid image area between the case of 50% duty ratio and the case of 100% duty ratio is reduced as shown in FIG. 3(a). In this condition, when the surface potential curve according to which the image carrier shown in FIG. 1 is exposed, as shown in FIG. 3(b), the difference of the surface potential between the case of 50% duty ratio and the case of 100% duty ratio is .DELTA.VS.sub.1, which is large. That is, the surface potential in the case of 100% duty ratio is higher than that in the case of 50% duty ratio by .DELTA.VS.sub.1.
In contrast to such a condition, even when the electric charge is eliminated by the image exposure of the solid image in the case of the 50% duty ratio and in the case of the 100% duty ratio, the difference of the surface potential of the solid image is .DELTA.VS.sub.2, as shown in the drawing, which is very small. After the above-described charging, image exposure processing, and developing processing are conducted, when the image forming processing shown by FIGS. 1(c), (d), (e), (f) is conducted, the condition of electric charge elimination by the imagewise exposure with a 50% duty ratio, is shown in FIG. 3(c). In the dot area and the solid image area, since the potential drop of the dot area is small, when developing is conducted by the developing unit D as shown in FIG. 4(a), an adhered amount of toner is very small, and since the potential drop of the solid image area is large, toner is fully adhered to the area. Next, charging is conducted again on the toner image in the first color development as shown in FIG. 4(b), and the second imagewise exposure of 100% duty ratio is conducted for the second color development as shown in FIG. 4(c). In this case, since the adhered amount of toner in the dot area is small on the image carrier E2 when the first color is developed, large energy is projected when the second color is imagewise exposed, and therefore, the potential is greatly lowered. Since the adhered amount of toner of the solid image area is large, and the projected amount of the optical energy is small when the second color is imagewise exposed, the potential drop is smaller than that of the dot area as shown in the drawings. Thus, when the duty ratio % is small, a large difference is produced in the lowering of the potential between the dot area and the solid image area. In this case, when the second color toner is developed by the developing unit E1, since the electric charge of the dot area is largely eliminated, a large amount of toner is adhered to the dot area compared to the solid image area. FIG. 4(d) shows the case where a duty ratio at the time of the second color imagewise exposure is larger than that at the time of the first color imagewise exposure (50%.fwdarw.100%). In this case, the adhered amount of the second color toner is much larger than that of the first color toner, and the color tone of the dot area is different from that of the solid image area, and therefore, color adjustment is necessary. In this case, there is a disadvantage in that the color tone of the dot area is different even when the adhered amount of the first color and that of the second color of the solid image area is almost equal and the desired color image is obtained.
In other color tone control methods, there is a method in which developing conditions are changed at each color and an amount of developing toner is controlled, however, in this method, the adhered amount of toner is different also depending on the image pattern, so that the color tone is changed. For example, in the developing method in which an A.C. voltage is impressed upon the developing bias voltage, an amount of the developing toner can be controlled by changing the value of the A.C. voltage. However, the following result has been found: when the A.C. voltage is increased, an amount of the developing toner of the dot or the line is greatly increased compared with that of the solid image. Accordingly, the color tone of the solid image is different from that of the dot or the line. Further, in the monochrome image, when the image density adjustment is conducted, there is a case where blocking, blurring, and breaking of the character are generated even when desired density of the solid image area is obtained. In color image reproduction, even when the desired color of the solid image was reproduced at the time of color adjustment, there were cases where the desired color of the edge area of the solid image, the dot, or the line was not reproduced.
Furthermore, when the color adjustment is conducted at the time of the color image reproduction, there is a disadvantage in that resolution is changed, the character becomes unclear, and thus the image quality is deteriorated.